1. Field of the Invention
The present invention relates to color correction in an image forming apparatus.
2. Description of the Related Art
A color image forming apparatus employing an electronic photographing method, such as a color printer and a color copier, is required to provide a further higher output image quality. In such a situation, a gray scale of image density and stability thereof are known to have a significant influence to human determination whether image quality is satisfactory or not.
For this point, an image density varies according to variation caused by environmental change and long-term use in each part of an apparatus, and, particularly in the case of the color image forming apparatus using the electronic photographing method, the density variation is caused even by a minimal environmental variation and there is a problem that color balance is broken. Accordingly, it is necessary to have a means for always maintaining a constant density-gray scale characteristic. Therefore, the color image forming apparatus is provided with a gray scale correction means such as a lookup table (LUT) for each toner. This gray scale correction means optimizes a process condition such as an exposure amount and a development bias and selects the most appropriate value for the gray scale correction according to absolute humidity measured by a temperature-humidity sensor. Further, density control (calibration) using a density sensor is also performed so as to obtain the constant density-gray scale characteristic even when the variation occurs in each part of the apparatus. This calibration generates a density detection patch using each color toner on an intermediate image transfer body, a drum, or the like, detects the density of the unfixed patch by the density sensor, and feeds back the detection result to the process condition such as the exposure amount and the development bias.
The above described calibration is performed by detecting the unfixed patch formed on the intermediate image transfer body, the drum, or the like. However, it is known that the color balance is also changed by an image transfer efficiency when a toner image is transferred onto a recording medium such as a paper and by heating and pressure application in fixing. Accordingly, the above calibration cannot accommodate up to the color balance change of an image caused by the image transfer to the recording medium and the fixing. Further, in the electronic photographing method, even when the density correction is performed for a single color of cyan (C), magenta (M), yellow (Y), or black (K) and each density is stabilized, sometimes a color deviation is caused in a mixed color of a combination thereof.
Accordingly, there is an image forming apparatus including a sensor (color sensor or spectrum sensor) that detects density, chromaticity, and spectrum reflectivity of the patch images on a recording medium after gray scale patches of single colors of CMYK and a mixed color patch thereof are formed and fixed on the recording medium. Such an image forming apparatus can perform more precise mixed color correction by a multi-dimensional LUT generated using values read out by the sensor. Further, manual read-out operation performed so far using a reader-scanner becomes unnecessary. Therefore, there is an advantage of executing calibration processing automatically for a user who cannot secure a manager or the like, a user who wants to reduce operator cost (human cost) and a user who does not have sufficient knowledge for the correction. In this manner, the mixed color correction using a sensor is expected to provide a more precise result than the single color correction and has an advantage of performing the calibration without human work.
On the other side, there has been a problem that the mixed color correction using a sensor needs a longer time for the correction than the single color correction and consumes a lot of paper and toner for patch pattern printing. For example, a user who mostly treats typical office documents and seriously considers cost reduction, desires to restrict execution of the mixed color correction, which consumes a lot of toner and time, as far as possible.
For satisfying such a desire, a technique is proposed as disclosed in patent document 1, for example. Japanese Patent Laid-Open No. 2005-321570 discloses a technique realizing color correction in a desired preciseness by enabling a combination in a patch group, which is to be read out by a sensor, to be changed according to a user's instruction in an image forming apparatus using a mixed-color multi-dimensional LUT, for suppressing excessive consumption of resources and time.
However, in the technique described in Japanese Patent Laid-Open No. 2005-321570, an operator needs to designate a color correction preciseness, and the user's desire to reduce the human cost is not satisfied sufficiently. Further, sometimes the color correction preciseness designated by the operator does not fit job contents to be actually utilized.